1. Field of the Invention
The present invention relates to a compact inner focus lens having high imaging performance.
2. Description of the Related Art
Conventionally, a long flange focal length relative to the focal length has to be established and thus, many lenses for single-lens reflex cameras adopt a configuration that includes a positive lens group toward the rear of the optical system to easily establish back focus. Nonetheless, in recent years, camera bodies have decreased in size and consequent to the spread of digital cameras, instances where a long flange focal length is not necessary are increasing.
Further, since video filming is also possible by a digital camera, high-speed autofocus processing for video filming is desirable. A portion of a lens group (focusing group) is moved rapidly along the optical axis (wobble) to achieve transitions: a non-focused state→focused state→non-focused state. A signal component of a specific frequency band of a partial image area is detected from the output signal of the image sensor; an optimal position of the focusing group achieving a focused state is determined; and the focusing group is moved to the optimal position. In particular, with video filming, this series of operations has to be rapidly continued, repeatedly. Further, in the execution of wobble, rapid driving of the focusing group has to be possible and the focusing group is demanded to have the smallest diameter possible and to be light-weight.
To address such demands, an inner focus lens that can also sufficiently cope with video filming has been proposed (for example, refer to Japanese Patent Application Laid-Open Publication No. 2013-97212).
The inner focus lens disclosed in Japanese Patent Application Laid-Open Publication No. 2013-97212 has a medium telephoto focal length by a 35 mm film camera conversion and a small, light-weight focusing group internally, thereby enabling favorable wobble to be executed.
On the other hand, conventionally, at the image sensor that optically receives and converts an optical image into an electronic image signal, there are limitations for efficiently taking in incident light by the on-chip microlens, etc. and on the lens side, the exit pupil is made to be greater than a certain diameter and assured telecentricity of the luminous flux incident to the image sensor is desirable.
Nonetheless, with recent image sensors, improved aperture ratios and advances in the design freedom of on-chip microlenses have reduced the exit pupil limitations demanded on the image lens side. Furthermore, with recent software and camera system advances and improvements, even when distortion is significant to an extent that conventionally, the distortion would be conspicuous, correction by image processing has become possible.
Therefore, in conventional image lenses, although a positive lens element is disposed farthest on the image side of the optical system and telecentricity is assured, in recent years, this is no longer necessary and even when a negative lens element is disposed farthest on the image side of the optical system and there is oblique incidence of the luminous flux on the image sensor, vignetting (shading) consequent to mismatching of the on-chip microlens and pupil, etc. has become inconspicuous. Further, since a negative lens element can now be disposed farthest on the image side of an optical system, reductions in the diameter of optical systems can be expected.
In contrast, with the inner focus lens disclosed in Japanese Patent Application Laid-Open Publication No. 2013-97212, since a positive lens element is disposed farthest on the image side of the optical system, which has a shorter overall length, the diameter of the third lens group (lens farthest on the image side) cannot be sufficiently reduced. Therefore, provision of an inner focus lens for cameras having a smaller dimension along the direction of the diameter of the optical system is difficult, including for mirrorless interchangeable-lens cameras that have come into wide use. Furthermore, when focusing is performed, to suppress aberration variations consequent to wobble and the effects of magnification, disposal of a negative element farthest on the image side of the optical system is desirable.
The inner focus lens disclosed in Japanese Patent Application Laid-Open Publication No. 2013-97212 is not aimed for wide angle views and therefore, the correction of field curvature and distortion as well as assuring the amount of light at lens edges necessary for wide angle views are points that have not been considered.